Effects of Strengthening the Masonry Infills by Reinforced Shotcrete on Seismic Behavior of Steel Frames with Khourjeeni Connections and their Elements

Steel frames with Khorjeeni connections have been widely used in the traditional construction of buildings in Iran during the past decades. In the traditional form of Khorjeeni connections، double section beams are not cut at the intersection with columns، rather they are connected to the column by means of two angles placed over the top and under bottom of the beam flanges. This type of connection offers advantages for frames، which carry gravity loads، but it has deficiencies when the frames are subjected to lateral loads. Like other structural frames، there are masonry infills in many frames with Khorjeeni connections. The behavior of composite frames subject to lateral loads differs from that of bare steel frames. In this paper، positive and negative effects of masonry infills، when strengthed by reinforced shotcrete، are studied on the behavior of steel frames with Khorjeeni connections. Finite element method was used to carry out nonlinear static analysis of subassemblages of this type of frames. Initially، the results of some experiments were utilized to verify the details of the model. Then numerical models of two span or four span frames with different configuration of bracings and masonry infills and characteristics of shotcrete were studied. The results showed that infills increase the stiffness and strength of frames in the absence of bracing considerably. Even when bracings are present، the increase in stiffness and strength is significant. When the infills are strengthened، their effects on stiffness and strength of composite frames increase. However if the steel frames are not strong enough، their strength limit the effects of infills. The masonry infills، however، have some negative effects on the behavior of Khorjeeni frames. Parts of the column in the vicinity of connections are prone to plastic damage، particularly when the infills are relatively strong. The Khorjeeni connections are subjected to vertical forces and tortional moments. Due to limited vertical strength of these connections، top stories of this type of frames may suffer when compressive action of strut is mobilized. For other bays، this action introduces additional moments، which may damage the connections. Therefore، considering these negative effects of infills is very important when seismic behavior of the existing frames is assessed. When the infills are strenghted by shotcrete، these negative effects become even more important and the inter story drift rations need to be limited in order to avoid failure in conections.